I think Necromancer should be able to resurrect for a while after receiving lethal damage but resurrect as a ghost warrior or as a shadow warrior and fight for 5 to 10 seconds, and then die. Necromancer should use illness and diseases because dead bodies rot and spread disease. Also Necromancer should be a summoner of undead creatures that instead of summoning one pet indefinitely it should summon many pets for short amount of time, and dealing great damage means keeping maximum number of summons as much time as possible (pets like zombies, skeletons, etc.). It would be great if necromancer have mechanics with the summoned pets (maybe while pets are fighting or with the bodies they leave when die), something like explode a body and create diseases and stun, consume a body and restore health, consume a body and buff the party (including pets), consume a pet to summon a better creature, etc.
Remember not to confuse witch with necromancer, necromancer use necrotic magic, while witches use demonic magic. In other words necromancer plays with dead elements while witch sell her soul to a demon and use element taken from hell.